Low Level Measurements Handbook

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In order to cancel internal offsets, an offset or drift compensation circuitallows the preamplifier offset voltage to be measured during specificphases of the measurement cycle. The resulting offset voltage is subsequentlysubtracted from the measured signal to maximize measurementaccuracy.Once the preamplifier amplifies the signal, it’s converted to digitalinformation by the A/D converter. Digital data is then sent to the display andthe IEEE-488 interface.SMUsThe SMU provides four functions in one instrument: measure voltage, measurecurrent, source voltage and source current. Generally, such instrumentscan simultaneously source voltage and measure current or simultaneouslysource current and measure voltage.When configured to Source I and Measure V (as shown in Figure 1-29),the SMU will function as a high impedance current source with voltagemeasure (and voltage limit) capability.Selecting either local or remote sense determines where the voltagemeasurement will be made. In local sense, the voltage is measured at theoutput of the SMU. In remote sense, the voltage is measured at the deviceunder test, eliminating any voltage drops due to lead resistance.The driven guard (×1 Buffer) ensures that the Guard and Output HI terminalsare always at the same potential. Proper use of Guard virtually eliminatesleakage paths in the cable, test fixture, and connectors. When configuredto Source V and Measure I (as shown in Figure 1-30), the SMU willfunction as a low impedance voltage source with current measure (and currentlimit) capability.SourceMeter InstrumentLike an SMU, a SourceMeter instrument can source current, source voltage,measure current and measure voltage. However, the SourceMeter instrumentalso has a sixth terminal, guard sense, which allows making moreaccurate measurements of networks. When configured to source current asshown in Figure 1-31, the SourceMeter unit functions as a high impedancecurrent source with voltage limit capability and it can measure current, voltage,or resistance.For voltage measurements, the sense selection (two-wire local or fourwireremote) determines where the measurement is made. In local sense,voltage is measured at the IN/OUT terminals of the instrument. In four-wireremote sense, voltage is measured directly at the device under test using theSense terminals. This eliminates any voltage drops due to lead resistance.When configured to source voltage as shown in Figure 1-32, theSourceMeter instrument functions as a low impedance voltage source withcurrent limit capability and it can measure current, voltage, or resistance.<strong>Low</strong> <strong>Level</strong> DC Measuring Instruments 1-31
FIGURE 1-29: Source I Mode of SMU×1BufferGuardOutput HIGuardLocalRemoteSense HIGuardI SourceV MeterRemoteLocalSense LOOutput LOOutput LOFIGURE 1-30: Source V Mode of SMUI MeterOutput HIGuardLocalRemoteSense HIV SourceMeasur eOutputAdjustV Source(Feedback)V MeterRemoteGuardSense LOLocalOutput LOOutput LO1-32 SECTION 1
Page 1 and 2: www.keithley.comLLM6 th EditionLow
Page 4 and 5: TABLE OF C ONTENTSSECTION 1 Low Lev
Page 6 and 7: 3.3 Low Resistance Measurements....
Page 8 and 9: SECTION 1Low Level DCMeasuringInstr
Page 10 and 11: 1.1 IntroductionDC voltage, DC curr
Page 12 and 13: FIGURE 1-3: Typical Digital Multime
Page 14 and 15: Coulombmeter FunctionCurrent integr
Page 16 and 17: 1.3.6 The SourceMeter ® Instrument
Page 18 and 19: TABLE 1-1: Specification Conversion
Page 20 and 21: Transfer StabilityA special case of
Page 22 and 23: FIGURE 1-6: Common Mode NoiseMeasur
Page 24 and 25: FIGURE 1-8: Voltage Amplifier+-V 2A
Page 26 and 27: Picoammeter amplifier gain can be c
Page 28 and 29: Using a small-signal transistor in
Page 30 and 31: FIGURE 1-17: High Resistance Measur
Page 32 and 33: FIGURE 1-20: High Resistance Measur
Page 34 and 35: Micro-ohmmeterThe micro-ohmmeter al
Page 36 and 37: FIGURE 1-26: Typical Digital Electr
Page 40: Sense circuitry is used to monitor
Page 43 and 44: 2.1 IntroductionAs described in Sec
Page 45 and 46: cause significant loading errors. I
Page 47 and 48: Cable leakage resistance is a commo
Page 49 and 50: FIGURE 2-6: Guarding Leakage Resist
Page 51 and 52: Example: Assume R S = 10GΩ and C S
Page 53 and 54: MaterialTABLE 2-2: Properties of Va
Page 55 and 56: QuartzQuartz has properties similar
Page 57 and 58: FIGURE 2-11: Guarding as Applied to
Page 59 and 60: FIGURE 2-13: Guarding the Leakage R
Page 61 and 62: FIGURE 2-15: Simplified Model of a
Page 63 and 64: time and/or temperature. Zero offse
Page 65 and 66: open-circuited, allow the reading t
Page 67 and 68: to equalize charges and minimize ch
Page 69 and 70: If insulators become contaminated,
Page 71 and 72: The input resistance of a feedback
Page 73 and 74: ence. However, in some cases, shiel
Page 75 and 76: FIGURE 2-27: Feedback Coulombmeter
Page 77 and 78: Advantages of Using a Coulombmeter
Page 79 and 80: FIGURE 2-30: Constant-Voltage Metho
Page 81 and 82: In addition to the voltage drop lim
Page 83 and 84: FIGURE 2-34a: Effects of Cable Resi
Page 85 and 86: Such devices require extreme care i
Page 87 and 88: charge will be lost through the zer
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FIGURE 2-40: Proper ConnectionCurre
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Figure 2-43 shows an example of AC
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fering voltage or current. A guard
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esponse is the rise time of the ins
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FIGURE 2-47: Shunt Capacitance Effe
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Resistance Measurements (Constant-C
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FIGURE 2-52: Noise Voltage vs. Band
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Source ResistanceAfter the bandwidt
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a. ConfigurationShieldCenterconduct
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• Shielding and Guarding: The fix
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floating circuits, a second grounde
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3.1 IntroductionLow voltage and low
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Thermoelectric EMFsThermoelectric v
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Reversing Sources to Cancel Thermoe
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quency spectrum of these interferen
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ment is almost entirely determined
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FIGURE 3-9: Low Voltages Generated
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FIGURE 3-11a: Multiple Grounds (Gro
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Common-Mode Reversal ErrorsReversin
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FIGURE 3-14: Two-Wire Resistance Me
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FIGURE 3-16: Canceling Thermoelectr
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Both V A and V B are affected by th
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If using a micro-ohmmeter or DMM to
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FIGURE 3-20: Dry Circuit Testing Us
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SECTION 4Applications
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For timing and integrating applicat
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FIGURE 4-2:Using an Electrometer to
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If current flows, the electrodes wi
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4.3 Low Current Measurement Applica
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DC. The Model 6517A can also be use
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voltage (V DS ) and measures the re
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The Keithley Model 248 High Voltage
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FIGURE 4-16: Ion Collector with Gro
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Figure 4-19 shows a Model 6430 conn
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FIGURE 4-21: SIR Test System to Mea
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FIGURE 4-22: Volume ResistivityElec
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these problems, the Alternating Pol
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FIGURE 4-25: Four-Point Collinear P
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Using two electrometers eliminates
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A plot of this function is shown in
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FIGURE 4-31: van der Pauw Measureme
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Through interactive programming, th
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objects. As discussed in Section 2.
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FIGURE 4-36: Faraday CupOutside Ele
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FIGURE 4-38: Connections for Standa
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FIGURE 4-40: Microcalorimeter with
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Measurement MethodFigure 4-42 illus
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FIGURE 4-43: Using a Nanovoltmeter
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exceed the critical current of the
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equation for V/I. Most materials ha
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FIGURE 4-49: van der Pauw Connectio
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5.1 IntroductionChoosing a specific
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TABLE 5-1a: Low Current/High Resist
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Table 5-1b: Source-Measure Instrume
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Table 5-2: High Speed Power Supplie
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Table 5-3: Connectors, Adapters, an
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Table 5-4: CablesTERMINATIONS LENGT
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Table 5-5: Test Leads and ProbesMOD
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Table 5-6: Switching Cards for the
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APPENDIX ALow LevelMeasurementTroub
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Measurement Type andTypical Applica
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Proper cable and connector assembly
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APPENDIX CGlossary
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channels. For matrix cards, a chann
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R. Buckminster Fuller. Sometimes ca
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NORMAL-MODE REJECTION RATIO (NMRR).
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SOURCEMETER. A SourceMeter instrume
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APPENDIX DSafetyConsiderations
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The types of product users are:Resp
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are the same. Other components that
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1/f noise, 3-7 to 3-83dB point, 2-5
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Normal mode rejection ratio (NMRR),
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Specifications are subject to chang
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Specifications are subject to chang
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